The present invention relates to a structure-borne sound detector for an intruder alarm system, with a structure-borne sound microphone, which has a piezoelectric sensor for structure-borne sound vibrations and for converting these into electrical signals, connected to, an electronic evaluation unit.
These structure-borne sound detectors, also described as noise detectors, are used to detect attacks on protective objects made of steel or concrete and on safes with plastic-reinforced protective coatings. The detectors are deployed in particular to monitor vault walls and doors, safes and automatic teller machines. Their mode of operation is based on the fact that when hard materials such as concrete or metal, for example, are disturbed, mass accelerations arise, due to which mechanical vibrations are generated, which disperse in the material as structure-borne sound. The piezoelectric sensor picks up these vibrations and converts them into electrical signals. The detector electronics evaluate the signals and trigger an alarm in the event of an appropriate evaluation result.
In all structure,borne sound detectors known today, the sensor is formed by a piezoelectric wafer, which is glued to its carrier. Due to this, the sensitivity of the microphone is influenced primarily by external parameters such as contact pressure, the thickness and quality of the layer of adhesive, by the surrounding mechanical components and the like. This has the consequence that each individual detector has to be adjusted, which increases production costs by a considerable amount.
A second consequence of the dependence of the microphone on external parameters consists in the fact that the transfer function of the microphone, i.e. the pick-up and transmission of the structure-borne sound, is not controllable or reproducible. Added to this is the fact that the resonance frequency and sensitivity of the structure-borne sound microphone is influenced very considerably by the metal flange forming part of the sensor casing.
The known structure-borne sound detectors are now to be improved by the invention in such a way that the sensitivity Of the microphone depends primarily on nothing but the sensor characteristic and no longer on external parameters, and thus no further trimming or adjustment is required. The transfer function of the microphone should also be reproducible and controllable.
The problem set is solved according to the invention in that the piezoelectric sensor contains a defined vibrating bar, which is formed by a bimorph element and attached by means of hybrid technology to a conductive carrier.
The sensor according to the invention has the principal advantage that the resonance frequency of the vibrating bar depends almost exclusively on its dimensions and only to a very small extent on the flange supporting the microphone. The sensitivity and resonance frequency of the microphone are thereby dependent exclusively on the sensor characteristic and no longer on external parameters, owing to which no further adjustment is necessary and production is accordingly simplified and reduced in cost.
Bimorph elements suitable for use in the invention, are described in the Philips Catalog "Piezokeramik" (Philips AG Components 1990/91) at pages 15.9, 15.19 and 15.20. Such bimorph elements consist of two thin wafers of piezoceramic material (denoted PXE wafers) joined together as a unit operable in an oscillation mode (e.g. longitudinal oscillation with transversal excitation). In a serial bimorph the PXE laminas are switched in series with the supply source, whereas, in a parallel bimorph each PXE lamina is driven by the supply source. In a serial bimorph one of the laminas is operated with a voltage acting in the opposite direction to the polarization voltage which increases the danger of depolarization. This may apply to a parallel bimorph also, but the latter can also be driven in another way. In this case both laminas are operated in the direction of polarization and, in this way, changes of the material characteristics by depolarization effects are avoided.
For serial bimorphs, the lamina with the highest R has the highest voltage and can depolarize when a voltage is acting in the opposite direction. For parallel bimorphs, both laminas have the same voltage, and depolarization is independent of the resistance. For actuator applications, parallel bimorphs are normally the most suitable ones. Serial bimorphs advantageously should be used in sensor applications.
The formation of the vibrating bar from a bimorph element has the advantage of creating a precisely reproducible connection between the bimorph element and the conductive carrier, due to which the manufacturing costs of the sensor are further reduced.
The conductive carrier for its part is attached to a base, preferably to a transistor header, so that an electrical connection exists between this and the lower part of the bimorph element.